1. Field of the Invention
This invention relates to a focus detecting system for use in camera apparatus such as video cameras, and more particularly to a focus detecting system for adjusting the focus of a camera system by making use of video signals.
2. Description of the Related Art
Generally, conventional focus detecting systems are classified as being of the active or passive types. While ultrasonic systems have been known as active type focus detecting systems, infrared systems of this type are generally employed these days, in which infrared rays are emitted from the camera to form a spot on the object, and the light reflected on the object is detected by a light receiving element, thereby effecting the focusing. This is because infrared systems display a comparatively high degree of distance-metering accuracy. However, infrared systems require a linking mechanism adapted for moving the light receiving element and so forth in accordance with the camera-to-subject distance. Therefore, they have complicated forms of construction and necessitate complicated adjustment steps, thus being defective in terms of operational efficiency.
With respect to the passive type focus detecting system, there have been systems which utilize video signals of the image pick-up system, and systems utilizing no video signal. Examples of the latter type of system are the Honeywell system, the Canon SST (Solid State Triangulation) system, etc. These systems also require optical parts that are not related to the fundamental function of the image pick-up system along with a high level of adjustment technique.
Focus detecting systems utilizing video signals are advantageous in that they do not need any optical parts other than the essential optical system.
A conventional focus detecting system utilizing video signals will be described with reference to FIG. 4. As shown in FIG. 4, an image formed by a pick-up lens assembly 1 constituting an image pick-up means is converted into an electric signal by a pick-up element 2 provided as an image pick-up means, and the signal thereby obtained is amplified by a preamplifier 3. The signal amplified by the preamplifier 3 is supplied as a video signal to a monitor or a video tape recorder (not shown) by way of a processing circuit 4. Simultaneously, the high band frequency components alone of this signal which are amplified by the preamplifier 3 are separated by a band-pass filter (hereinafter referred to as "BPF") 5 and are supplied through a gate circuit 6 which picks up only signals corresponding to a predetermined image plane area on which the image pick-up system is focused when the image is displayed on the monitor or the like, for instance, the general area of the center of the picked-up image, thereby restricting the distance-metering field. The separated signal is thereafter converted by a detection circuit 7 and an integration circuit 8 into a value which represents a focused state. These elements 3, 5, 6 and 7 constitute a signal processing means. The value representing a state of focusing (hereinafter referred as "focusing value") displays a relationship with the amount of defocus (the amount of deviation from the focus) of the image pick-up lens assembly 1 such as that shown in FIG. 5. In accordance with this relationship, a motor driving circuit 9 drives, through a motor, a focusing lens in the image pick-up lens assembly 1 to the position at which the focusing value is maximized.
Next, the operation of the motor driving circuit 9 as shown in FIG. 4 will be described with reference to FIG. 5. It is assumed that the focusing is at a point indicated by a reference symbol A in FIG. 5 at the time of the commencement of distance-metering, and that a reference symbol B indicates a focusing point displayed after the focusing lens in the image pick-up lens assembly 1 has been slightly moved by a predetermined pitch. The motor driving circuit 9 compares a focusing value f(A) at the position A supplied from the integration circuit 8 with a focusing value f(B) at the point B. If f(B)&gt;f(A), the driving circuit rotates the motor in the same direction as that in which the motor rotates to move the focusing lens from the point A to the point B. If f(B)&lt;f(A), the direction of the rotation of the motor is reversed, and the focusing lens in the image pick-up lens assembly 1 is thereby moved in the direction of the in-focus point X.sub.0.
However, the above-described conventional type of focus detecting system which utilizes video signals cannot discriminate the in-focus state until the focusing point passes through the in-focus point, and cannot obtain natural images picked up and formed. Moreover, when the state of focusing is deviated greatly from the in-focus state (called a greatly out-of-focus state), the differential focusing value f(n)-f(n-1) created by a slight amount of movement of the focusing lens is so small that the system mistakes the greatly out-of-focus state for the in-focus state.